Flutter StreamProvider 使用了一个 BuildContext ，它是提供者的祖先

Question

我正在用 Flutter 开发一个应用程序（我也还是个新手），但我遇到了以下错误：

Error: Could not find the correct Provider<List<Category>> above this Exercises Widget

This likely happens because you used a `BuildContext` that does not include the provider
of your choice. There are a few common scenarios:

- The provider you are trying to read is in a different route.

  Providers are "scoped". So if you insert of provider inside a route, then
  other routes will not be able to access that provider.

- You used a `BuildContext` that is an ancestor of the provider you are trying to read.

  Make sure that Exercises is under your MultiProvider/Provider<List<Category>>.
  This usually happen when you are creating a provider and trying to read it immediately.

  For example, instead of:

  ```
  Widget build(BuildContext context) {
    return Provider<Example>(
      create: (_) => Example(),
      // Will throw a ProviderNotFoundError, because `context` is associated
      // to the widget that is the parent of `Provider<Example>`
      child: Text(context.watch<Example>()),
    ),
  }
  ```

  consider using `builder` like so:

  ```
  Widget build(BuildContext context) {
    return Provider<Example>(
      create: (_) => Example(),
  // we use `builder` to obtain a new `BuildContext` that has access to the provider
  builder: (context) {
    // No longer throws
    return Text(context.watch<Example>()),
  }
),

我一直在网上查找，这显然与我在 Provider.of>(context) 中调用 Provider.of>(context) 时无法获得正确的 'context' 有关39=]，我真的不明白为什么。因为正如您在我的 exercises.dart 中看到的那样，我有 'body: ExerciseList()'，我可以在其中从 StreamProvider 获取类别，但是当我尝试通过单击我的 'floatingActionButton' 和然后尝试打开我的 ExerciseAdd() 页面，它抛出该错误。

我真的很感激关于如何修复我的代码以及为什么它不起作用的解决方案（+解释）。

exercises.dart

  Widget build(BuildContext context) {
    return _isLoading
        ? Loading()
        : MultiProvider(
            providers: [
              StreamProvider<List<Exercise>>(
                create: (context) => DatabaseService().exercises,
              ),
              StreamProvider<List<Category>>(
                create: (context) => DatabaseService().categories,
              ),
              ChangeNotifierProvider<ExerciseFilter>(
                create: (context) => ExerciseFilter(isActive: true),
              )
            ],
            child: Scaffold(
              appBar: AppBar(
                title: Text('Exercises'),
                elevation: 0.0,
                actions: _buildActions(),
              ),
              body: ExerciseList(),
              floatingActionButton: FloatingActionButton(
                backgroundColor: Colors.black,
                child: Icon(Icons.add, color: Colors.white),
                onPressed: () {
                  Navigator.push(
                     context,
                     MaterialPageRoute(
                       builder: (context) => ExercisesAdd(),
                     ),
                   );
                },
              ),
            ),
          );
  }
 }

exercises_add.dart

 @override
  Widget build(BuildContext context) {
    final cats = Provider.of<List<Category>>(context);
    print(cats.length);

    return Scaffold(
      appBar: AppBar(
        title: Text('Add Exercise'),
        elevation: 0.0,
      ),
      body: SingleChildScrollView(
        child: Container(
          padding: EdgeInsets.symmetric(vertical: 20.0, horizontal: 50.0),
          child: Form(
            key: _formKey,
            child: Column(
              children: <Widget>[
                SizedBox(height: 20.0),
                TextFormField(
                  decoration:
                      textInputDecoration.copyWith(hintText: 'Exercise Name'),
                  validator: (value) {
                    if (value.isEmpty) {
                      return 'Exercise name is required';
                    }

                    return null;
                  },
                  onChanged: (value) {
                    setState(() {
                      exerciseName = value;
                    });
                  },
                ),
                SizedBox(height: 20.0),
                Theme(
                  data: Theme.of(context).copyWith(canvasColor: Colors.white),
                  child: DropdownButtonFormField<String>(
                    decoration: dropdownDecoration,
                    value: exerciseCategory,
                    onChanged: (value) {
                      setState(() {
                        exerciseCategory = value;
                      });
                    },
                    items: categories.map<DropdownMenuItem<String>>((value) {
                      return DropdownMenuItem<String>(
                        value: value.name,
                        child: Text(value.name),
                      );
                    }).toList(),
                  ),
                ),
                SizedBox(height: 20.0),
                RaisedButton(
                  elevation: 0,
                  color: Colors.black,
                  child: Text(
                    'Add Exercise',
                    style: TextStyle(color: Colors.white),
                  ),
                  onPressed: () async {
                    if (_formKey.currentState.validate()) {
                      bool failed = false;
                      String uid = await _auth.getCurrentUser();

                      if (uid != null) {
                        dynamic result = DatabaseService(uid: uid)
                            .addExercise(exerciseName, exerciseCategory);
                        if (result != null) {
                          Navigator.pop(context);
                        } else {
                          failed = true;
                        }
                      } else {
                        failed = true;
                      }

                      if (failed) {
                        setState(() {
                          error = 'Failed to add exercise. Please try again';
                        });
                      }
                    }
                  },
                ),
                SizedBox(height: 12.0),
                Text(
                  error,
                  style: TextStyle(color: Colors.red, fontSize: 14.0),
                ),
              ],
            ),
          ),
        ),
      ),
    );

DatabaseService().exercises

List<Category> _categoryListFromSnapshot(QuerySnapshot snapshot) {
    return snapshot.documents.map((doc) {
      return Category(name: doc.data['category'] ?? '');
    }).toList();
  }

  Stream<List<Category>> get categories {
    return categoryCollection
        .orderBy('category')
        .snapshots()
        .map(_categoryListFromSnapshot);
  }

注意：StreamProvider 和 MultiProvider 等都是 'provider' 包的一部分（我使用的是最新版本）

Answer 1

您收到的错误描述了您当前所处的场景。

- The provider you are trying to read is in a different route.

Providers are "scoped". So if you insert of provider inside a route, then other routes will not be able to access that provider.

您正在导航到另一条路线并尝试访问提供商，但它不再位于小部件树中。

您只需将 MultiProvider 移动到您在小部件树中使用的任何导航器上方。您可能使用 MaterialApp 来执行此操作，因此移动 MultiProvider 并使 MaterialApp 成为 child.

Flutter StreamProvider 使用了一个 BuildContext ，它是提供者的祖先

Flutter StreamProvider used a `BuildContext` that is an ancestor of the provider

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